Use direct pointers for Symbols

[rpjohnst]

This relies on the extensions to Rust's consts described in issue
rust-lang/const-eval#11, and will not compile today.

[rpjohnst]

The idiom used here to emulate a C-style flexible array member (struct Entry { ..., data: [u8; 0] }) may be unsound. Entry::borrow forms a reference to the whole allocation from another reference that only covers the Entry itself. In Stacked Borrows terms, I believe this new reference will not have any granting items on the stack(s) for the actual data.

I'll need to revisit this as well before taking this approach. (EDIT: Or, Stacked Borrows may be tweaked to accommodate this: rust-lang/unsafe-code-guidelines#134.)